The anti-Markovnikovhydration of terminal alkynes to give aldehydes is catalyzed by complexes derived in situ from air-stable [CpRu(eta6-naphthalene)]PF6 (C) and 6-aryl-2-diphenylphosphinopyridines (L). Ligands L are readily available from a modular synthesis. Increasing the size of the ligand C-6 aryl group in the order R = Ph < mesityl < 2,4,6-triisopropylphenyl < (2,4,6-triphenyl)phenyl gave hydration
others. The scalable syntheses proceed in a few steps. The incorporation of AZARYPHOS ligands (L) into complexes [RuCp(L)2(MeCN)][PF6] (Cp=cyclopentadienyl) gives catalysts for the anti‐Markovnikov hydration of terminalalkynes of the highest known activities. Electronic and steric ligand effects modulate the reaction kinetics over a range of two orders of magnitude. These results highlight the importance